1. Field of the Invention
This invention relates to a projecting device capable of projecting on a light receiving surface an image of the surface of a body and continuously varying the focussing magnification of the image, and more particularly to a projecting device having an optical system capable of maintaining substantially constant the quantity of light even if the surface of the body is projected on a light receiving surface while the focussing magnification of the image is continuously varied.
2. Description of the Prior Art
Copying apparatus is widely known as a projecting device, which scans the surface of a body by a slit and continuously projects on the light receiving surface the slit image continuously. In conventional devices, when the image of the surface of a body to be projected on the light receiving surface is varied with its magnification, the intensity of illumination is varied on the light receiving surface so that in a copying apparatus where a photosensitive member is disposed at the light receiving surface, there occurs excess exposure or shortage of exposure. For compensating the change in amount of exposure, there have been proposed various methods, for example, varying the aperture of a stop as the magnification varies, or varying the width of the slit for scanning. However, these methods are mechanical so that the apparatus becomes complex and bulky.
Instead of the above mentioned compensation methods, U.S. Pat. No. 4,171,904 discloses a variable power copying apparatus which, without using a mechanical method, compensates the amount of exposure at the photosensitive surface. In this apparatus, the width of a mirror disposed near to an original to be copied is adjusted to maintain the exposure amount at the photosensitive surface approximately constant. In precise meaning, as described later, it is not possible, in principle, to obtain a constant exposure amount, so that fine control for maintaining constant exposure amount is not expected.